The invention relates to digital satellite communication systems with time-division multiple access for frequency re-use multiple spot-beam uplinks, an on-board satellite routing switch, and multiple spot-beam downlinks.
Among the more promising future satellite communications systems are those which will serve a large number of earth stations by a type of time-division multiple access transmission in which the uplink and the downlink each comprise a plurality of narrow spot-beams which may be frequency-channelized. Such spot-beams permit the re-use of the allocated frequency band by angular distribution of the individual beams to provide mutual spatial isolation of their signals. The signals are in digitally encoded burst form, and the routing of the uplink signals to their intended downlink location is done on-board the satellite by means of an electronic digital routing switch which processes the signals before they go to the transmitter for the downlink transmission. The satellite may also be capable of on-board regeneration of the signals and the individual transponders may include a variable antenna pointing, or spot-beam scanning feature.
While the satellites of such a system are themselves costly elements, as the traffic handling capability of the satellites become greatly increased, the number of earth stations served by a satellite becomes so large that the earth stations also become a very significant factor in the cost of the entire system. The earth stations must have a highly sensitive receiver to reliably receive the often very weak downlink signals and must also have strong transmitters with multiple frequency channel transmitting capability at a power level sufficient to reliably transmit the signals to the satellite.
It is generally recognized that from the standpoint of reducing the cost of the satellite, it is desirable to reduce the number of downlink transmitters, since each one requires an antenna port and a traveling wave tube. The traveling wave tube is relatively bulky and heavy. The number of transmitters can, of course, be reduced by minimizing the number of frequency-divided channels.
In one approach this is achieved by having a single wideband transmitter for each of the spot-beams, with no frequency-divided channels in the uplink or downlink beams. However, such an arrangement places a burden on the earth stations. Two factors enter into this. First, the transmission rate for such an arrangement is very high--from 600 to 800 Mb/sec. (million bits per second), if quadriphase modulation is employed. The second, and more important consideration is that to maintain the same energy per bit and bit error rate performance, each ground station must be equipped with a transmitter capable of providing perhaps 10 dB (decibels) more power, although at only 1/10 the duty cycle, as compared to a system employing 10 to 1 channelization.